![](https://csdnimg.cn/release/download_crawler_static/14933396/bg1.jpg)
Characteristics of optical emission during laser-induced
damage events of fused silica
Chao Shen (沈 超)
1,2,
*, Nanjing Zhao (赵南京)
1,2
, Jaka Pribošek
3
, Mingjun Ma (马明俊)
1,2
,
Li Fang (方 丽)
1,2
, Xingjiu Huang (黄行九)
4
, and Yujun Zhang (张玉钧)
5,
**
1
Key Laboratory of Environmental Optics & Technology, Anhui Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Hefei 230031, China
2
Key Laboratory of Optical Monitoring Technology for Environment, Hefei 230031, China
3
Carinthian Tech Research, 9524 Villach, Austria
4
Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, China
5
State Key Laboratory of Pulsed Power Laser Technology, National University of Defense Technology,
Hefei 230037, China
*Corresponding author: cshen@aiofm.ac.cn; **corresponding author: yjzhang@aiofm.ac.cn
Received August 8, 2019; accepted August 30, 2019; posted online December 2, 2019
Spontaneous optical emission properties of laser-produced plasma during laser damage events at input and exit
surfaces of fused silica were retrieved and compared. We show that plasma at the input surface is much larger in
size and exhibits significantly higher electron number density and excitation temperature, even when smaller
laser energy was used. This effect was attributed to the stronger laser–plasma coupling at the input surface.
In addition, a strong continuum background containing three peaks at 1.3 eV, 1.9 eV, and 2.2 eV was observed
at the exit surface, and possible origins for this effect are also discussed.
OCIS codes: 300.6365, 140.3330, 140.3440.
doi: 10.3788/COL201917.123002.
During many pulsed laser-induced damage (LID) events,
the incident laser intensity can be high enough to transfer
adequate energy to the target to produce a vaporized layer
and cause atomization and ionization processes in vapor.
The ionized vapor can absorb incident laser energy to
further increase its temperature and subsequently induce
the breakdown of vapor and the formation of plasma
[1,2]
.
This laser-produced plasma (LPP) has a strong and spon-
taneous broadband optical emission and exhibits as a
bright fireball
[3–6]
. Due to its intrinsic characteristics of
high pressure, temperature, and electron number density,
LPP usually produces additional effects on LID sites, such
as causing a burning scar around the LID site on optical
coatings
[7,8]
and assisting the formation of periodical struc-
tures on the target surface by facilitating the interference
between incident laser beams and beams reflected by the
plasma
[9,10]
. The role of LPP is particularly important in
LID of transparent optical components because the laser
can penetrate the component and generate plasma plumes
at either the input or exit surface
[11]
. For input surface
damage, LPP can directly interact with the incident laser
pulse, and the laser energy is mainly deposited in LPP. By
contrast, the laser pulse first interacts with the target bulk
during exit surface damage, and the laser energy is mainly
deposited in bulk material. Although the LPP is one of the
most obvious manifestations of this disparity between
input and exit surfa ce damage, limited works have been
reported focusing on LPPs formed on different surfaces.
Salleo et al.
[12]
have compared the propagation of shock
waves formed by the expansion of air part LPPs in input
and exit surface damage events of fused silica induced by a
35 ps infrared pulse. It was shown that the input surface
shock wave is stronger, and the deduced driving energy is
more than twice that of the exit surface one. Liu et al.
[13]
have observed similar results in the same target under the
irradiation of a 6.8 ns ultraviolet pulse. They attributed
this asymmetry to different expansion pressures of LPPs.
These results, however, are only based on absorption or
refraction techniques such as shadowgraphy and the
laser-deflection method and do not involve emission prop-
erties of LPP. Zeng et al.
[14–17]
investigated the emission
spectra of input surface plasma produced by 266 nm nano-
second pulses on fused silica. Using the Stark broadening
effect of a 288.16 nm Si I line and the line-to-continuum
ratio method, they found that the electron number
density and temperature of plasma could reach 10
18
cm
−3
and 10
4
K. However, they only performed damage on the
input surface, and they mainly focused on the comparison
between plasma produced on the flat surface and the
cavity structure. Raman et al.
[18]
have performed the pio-
neer work focusing on the emission spectra of the
exit surface LPP induced by 355 nm nanosecond pulses.
They recorded a continuum background in the visible
range superimposed with two peaks from nitrogen and
Si; however, in their work, no further details concern-
ing the plasma parameters were discussed. Recently,
Harris et al.
[19]
presented a more detailed discussion about
the exit surface plasma, and they deduced an electron
number density of around 10
17
cm
−3
and plasma temper-
ature of higher than 10
4
K. However, plasmas investigated
in this work were produced on the surface attached with
steel microspheres, and thus, obvious plasma confinement
COL 17(12), 123002(2019) CHINESE OPTICS LETTERS December 2019
1671-7694/2019/123002(6) 123002-1 © 2019 Chinese Optics Letters